Multilayer compositions have been utilized for many years. The concept of a multilayer is that the positive properties of two or more materials are combined with the structural integrity of each material being essentially uncompromised. Usually the positive properties of one material off-set or counter balance the weaker properties of the second material. For example, among the positive propertie of polycarbonate are high heat resistance and impact strength. However polycarbonate has a relatively high transmission rate for certain gases, oxygen and carbon dioxide for example. Polyvinylidene chloride does not have very high resistance to impact or heat but has excellent resistance to the transmission of oxygen and carbon dioxide. Therefore a multilayer composition utilizing polycarbonate adjacent to polyvinylidene chloride could be employed in structures wherein the properties of high impact resistance, high heat resistance and high resistance to oxygen and carbon dioxide transmission are necessary.
Although many of these multilayer compositions can be hypothesized on the basis of laminating a material possessing certain strong properties with a material having weaknesses in those same property areas, certain practical considerations inhibit successful implementation of this theory.
For example, today's modern society requires materials and structures which can rapidly go from one temperature extreme to another without showing structural defects such as cracking, brittleness, lack of rigidity, modulus and impact strength. These properties and others are of particular significance in designing a food carrying container which is able to go from the freezer to the high temperature conventional oven in a period of a few seconds. Of perhaps even greater interest is the ability of the container to be "dual ovenable", that is to function not only in the conventional oven fueled by gas or electricity but also in the microwave oven. In the latter case the container structure must not only be resistant to heat but it must also be transparent to the microwave energy.
It has now been discovered that certain specific multilayer structures will have the desired property mix and have strong inter layer adherence to each other without the oftentimes observed interfacial instability even though the viscosities of the adjacent resin layers are widely divergent.